The response is typically sending diplomats home, cancelling agreements, and sanctions. Something proportional. Not immediate outright war.

Romania has reluctantly done exactly that after the drone incident and tensions have escalated. The Romanians closed the Russian consulate in Constanta as a response and sent the consul back to Russia. The Russians sent an hijacked Ukrainian drone that blew up after 5 hours in the port of Constanta just after this. Now they're slowly escalating by periodically sending drones into Romanian airspace or attacking cargo ships bound from/to Odessa while in the Romanian EEZ.